This disclosure relates generally to seismic streamers/seismic cables for use in marine seismic surveys, and more specifically but not by way of limitation to streamers with a solid/rigid core and anti-biofouling properties.
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A seismic survey typically involves deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations creating pressure changes and vibrations along their way. Changes in elastic properties of the geological formation scatter the seismic waves, changing the direction of propagation and other properties of the seismic waves. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (e.g., hydrophones), others to particle motion (e.g., geophones), and/or the like. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
In a marine seismic survey, the equipment for which and arrangement of such equipment is depicted in FIGS. 1A and 1B, a survey vessel tows an array of seismic cables, frequently referred to as a “streamer,” along a predetermined course. As the vessel tows the array, a seismic source, such as an airgun or a vibrose is source, imparts an acoustic wave into the water. The acoustic wave travels through the water and is eventually reflected by various geological features. The reflections travel back up through the water to the streamers and these reflections are referred to herein as a seismic signal.
The streamers include sensors for detecting/determining properties of the reflections/seismic signals. These sensors may include acoustic sensors, or “hydrophones,” distribute along the length of the streamer. In a seismic survey, the acoustic receivers may be used to sense the magnitude of the passing wavefront of the reflections/seismic signals. The acoustic receivers then transmit data representing the detected magnitude of the passing wavefront back up the seismic cables to the survey vessel for collection. The streamer may comprise other types of sensors for sensing other properties of the seismic signals.
The survey system 100 may include an array 103 towed by a survey vessel 106 on board of which is a computing apparatus 109. The towed array 103 may comprise multiple marine seismic cables, or streamers, 112 (only one is shown in the figure) that may, for instance, each be of the order of multiple kilometers in length. Note that the number of streamers 112 in the towed array 103 is not material to the practice of the invention. Thus, alternative embodiments may employ different numbers of streamers 112.
A seismic source 115, typically an air gun or an array of air guns, is also shown being towed by the seismic survey vessel 106. Note that in alternative embodiments, the seismic source 115 may not be towed by the survey vessel 106. Instead, the seismic source 115 may be towed by a second vessel (not shown), suspended from a buoy (also not shown), or deployed in some other fashion known to the art. The known seismic sources include impulse sources, such as explosives and air guns, and vibratory sources which emit waves with a more controllable amplitude and frequency spectrum.
At the front of each streamer 112 is a deflector 118 (only one indicated) and at the rear of every streamer 112 is a tail buoy 120 (only one indicated). The deflector 118 horizontally positions the front end 113 of the streamer 112 nearest the seismic survey vessel 106. The tail buoy 120 creates drag at the tail end 114 of the streamer 112 farthest from the seismic survey vessel 106. The tension created on the streamer 112 by the deflector 118 and the tail buoy 120 results in a roughly linear shape of the streamer 112, shown in FIG. 1B.
Located between the deflector 118 and the tail buoy 120 are a plurality of seismic cable positioning devices known as “birds” 122. The birds 122 may be located at regular intervals along the seismic cable, such as every 200 to 400 meters. The birds 122 are used to control the depth at which the streamers 112 are towed, typically of the order of a few meters.
Seismic surveys may be called “marine” surveys because they are conducted in marine environments. In the marine seismic survey, the streamers that are used comprise long cables that house various sensor networks and other devices useful in the acquisition of seismic data. Generally, the streamers comprise liquid-filled streamers where the liquid is selected to have advantageous properties for seismic signal acquisition. Often, the liquid filling the streamer comprises kerosene. However, a problem with kerosene filled streamers is that if the streamer is punctured the kerosene may leak out and pollute the marine environment. As a result, some streamers are fabricated with a gel filler, which may comprise a kerosene gel or the like, which addresses the problem of leaking. Liquid and gel filled streamers may be used to provide for good data acquisition from the marine environment surrounding the streamer and/or to provide that the streamer is flexible so it can be stored on a spool when not deployed from the seismic vessel.
Within the core of the streamer, transmission and power bundles are disposed continuously through a streamer section (a segmented portion./.section of a streamer cable). The transmission and power bundles are typically connected to electronics modules between the streamer sections through end-connectors. Also within a streamer section, there is a need to connect distributed sensors and (if present) sensor electronics by wires to transmit power and data to the electronics modules.
Previously, seismic streamers have in general comprised sensors, strength members and cabling all disposed within a polyurethane casing, which may be referred to as the streamer skin/streamer casing. The casing for seismic streamers, the streamer skin, in general, is and has previously been made from thermoplastic polyurethane (“TPU”). TPU is any of a class of a polyurethane plastic.
In general, TPU has been used as a streamer casing material because it has many useful properties, including elasticity, transparency, and resistance to oil, grease and abrasion. TPU comprises thermoplastic elastomers consisting of linear segmented block copolymers composed of hard and soft segments. TPU is formed by the reaction of: (1) diisocyanates with short-chain diols (so-called chain extenders) and (2) diisocyanates with long-chain bifunctional diols (known as polyols). With TPU, there is virtually an unlimited amount of possible combinations producible by varying the structure and/or molecular weight of the reaction compounds allowing for an enormous variety of different TPUs. The different TPU recipes have allowed for tuning the TPU's polymer structure to the desired final properties of the resulting TPU for use in seismic streamers. For example TPU may be tuned to have a desired resistance to abrasion, opacity, attenuation/lack of attenuation of seismic signals, noise and/or the like. Moreover, TPU is a hydrophilic material and this property has been useful for effectively encapsulating the liquid and gel fillers used in the streamers, where the liquid/gel, in general, comprises kerosene or the like.